It is well established that the strength and hardness of aluminum pistons are reduced from room temperature values at piston operating temperatures. Also, piston thermal expansion significantly changes piston shape. This makes it extremely difficult to design the skirts as effective low friction bearings. It is the purpose of the present invention to provide a piston which has an inexpensive and structurally sound heat pipe cooling feature which makes the piston skirts approximately isothermal and enhances cooling of the piston crown.
The heat pipe has two purposes. First, it is intended to make a piston assembly more isothermal to control thermal distortions. The reduced distortions make the piston skirts more effective as full-film bearings in a design the inventor and his associates are developing. Secondly, the heat piped piston arrangement is intended to reduce peak metal temperatures and therefore increase material strengths. This allows the heat piped pistons to be built with less weight and/or better durability than is possible with current pistons to improve engine performance.
A heat pipe is a sealed volume, often including a wick capillary arrangement, which contains a working liquid in equilibrium with its own vapor. Noncondensable gases are excluded from the volume. Since the only vapor in the volume is the vapor of the working liquid itself, the liquid interface is always simultaneously at its boiling and condensing temperature for the pressure in the volume. If any part of the internal surface area of the enclosure is cooler than the liquid surfaces elsewhere inside the heat pipe enclosure, this cooler surface will condense vapor upon itself. This condensation will lower the vapor pressure in the heat pipe volume. In response, evaporation will occur off of the liquid surfaces which are slightly warmer, and the vapor will flow hydrodynamically at tiny pressure drops to the condensing surface. This condensing surface will be rapidly heated by the latent heat of vaporization of the vapor which condenses upon it. Thermal equilibration between surfaces is rapidly achieved in this way.
Similarly, if a liquid contacting surface is hotter than other areas in the heat pipe volume, the hotter surface will evaporate liquid and be cooled by the heat of vaporization of the evaporated liquid. So long as the heat pipe enclosure surfaces stay wet, heat transfer rates within the heat pipe passage will be extremely high. The heat transfer rates inside a heat pipe volume are so long that for most analytical purposes a heat pipe may be analyzed as an isothermal volume.
The limiting heat transfer rate in a heat pipe is roughly proportional to the vapor pressure of its working liquid as a function of temperature. If the heat pipe is heated to the point where all the liquid in the heat pipe evaporates, heat transfer stops since the evaporation process ceases. Therefore, heat pipes must be designed so there is always some unevaporated liquid in them at their maximum operating temperature.
Heat transfer rates required to cool a piston with a heat pipe are possible with a wide range of working liquids, and some of these working liquids will operate with peak vapor pressures not much in excess of atmospheric pressure in the temperature range required (200.degree. to 400.degree. F.). For this reason, the heat pipe containing structure can be made to be thin, light and flexible if the proper working liquid is chosen for the heat pipe.
It is desirable that the heat pipe arrangement for a piston be thin, light, and flexible. Lightness is desirable because the piston is subjected to high inertial forces (of the order of several thousand G's in racing applications). Flexibility is desirable so that the heat piped passage can be fastened to the inside of the piston easily and inexpensively. A light flexible heat piped structure can be fastened to the inside of the piston skirts and the inside of the piston crown by means of a thin high conductance elastomer-glue type layer to have intimate thermal contact with the piston crown and the piston skirts. The elastomer can be made thin enough that thermal resistance across the elastomer layer is small. The elastomeric connection between the heat pipe structure and the piston structure eliminates (actually buffers) stress concentrations due to strain buildups, and eliminates the need for precise geometrical matching between heat pipe geometry and piston geometry. For a thin and light heat pipe structure, the strength of an elastomer bond is ample to hold the heat pipe in place in the presence of the inertial stresses to which the heat pipe will be subjected. The elastomeric bonding process between heat pipe and piston per se is also inexpensive and lends itself to high volume mass production techniques.
A practical and inexpensive heat piped piston can be built as follows.